1. Field of the Invention
The present invention relates to a device and a method for improving speed of change in number of gradations and, more particularly, to a device and a method suitable for a matrix-type display such as liquid crystal panel.
2. Description of the Related Art
Liquid crystal used in a liquid crystal panel changes in transmittance due to cumulative response effect, and therefore the liquid crystal cannot cope with a moving image that changes rapidly. Hitherto, in order to solve this disadvantage, a liquid crystal drive voltage applied at the time of gradation change is increased exceeding a normal drive voltage, thereby improving response speed of the liquid crystal. (See the Japanese Patent No. 2616652, pages 3 to 5, FIG. 1, for example.)
In the case where the liquid crystal drive voltage is increased as described above, when increasing number of display picture elements in the liquid crystal panel, image data for one frame written in an image memory, in which inputted image data are recorded, increase. This brings about a problem that a large memory capacity is required. In order to reduce the capacity of the image memory, picture element data are skipped and recorded on the image memory. Then, when reading out the image memory, picture element data same as the recorded picture element data are outputted for the picture elements of which picture element data are skipped in several prior arts. (See the Japanese Patent No. 3041951, pages 2 to 4, FIG. 2, for example.)
As described above, when number of gradations in one frame that is displayed (this frame is hereinafter referred to as a display frame.) changes that in the other frame which is one frame previous to the display frame, the gradation change speed of the liquid crystal panel is improved by increasing a liquid crystal drive voltage applied at the time of displaying the display frame so as to exceed the normal liquid crystal drive voltage. However, in the case of the prior arts described above, the liquid crystal drive voltage to be increased or decreased is determined only on the basis of number of gradations in the display frame and that in the frame which is one frame previous to the display frame. As a result, in the case where the liquid crystal drive voltage includes any liquid crystal voltage corresponding to any noise component, the liquid crystal drive voltage corresponding to the noise component is also increased or decreased, which results in deterioration of image quality of the display frame. Particularly in the case of a liquid crystal drive voltage of which gradation minutely changes from the frame, which is one frame previous to the display frame, to the display frame, the liquid crystal drive voltage corresponding to the noise component is influenced more seriously than the case where the gradation changes largely, and image quality of the display frame tends to deteriorate.
In the case where capacity of the memory is reduced by skipping the image data stored in the image memory, the voltage is not properly controlled at the portion where the image data have been skipped. As a result, data of any portion, of which line is thin, such as contour of any image or characters are skipped. Thus, a problem exists in that image quality is deteriorated due to unnecessary voltage being applied. Another problem exists in that effect of improvement in the gradation change speed in the liquid crystal panel is decreased due to necessary voltage not being applied.